Bioethanol production for use as a liquid motor fuel is increasing worldwide. Such biofuels include, for example, ethanol that can be blended with gasoline with a wide range of compositions. One of the major drivers for bioethanol is its derivation from renewable resources by fermentation and bioprocess technology. Conventionally, biofuels are made from readily fermentable carbohydrates such as sugars and starches. For example, the two primary agricultural crops that are used for conventional bioethanol production are sugarcane (Brazil and other tropical countries) and corn or maize (U.S. and other temperate countries). The availability of agricultural feedstocks that provide readily fermentable carbohydrates is limited because of competition with food and feed production, arable land usage, water availability, and other factors. Consequently, lignocellulosic feedstocks such as forest residues, trees from plantations, straws, grasses and other agricultural residues are looked to as feedstocks for biofuel production. Unlike utilization of fossil fuels, deriving bioethanol from such plant or even municipal waste sources provides an environmentally sustainable resource for the production of liquid fuels.
A highly efficient route to the production of bioethanol is the gasification of biomass or other organic matter into a substrate gas comprising CO and/or hydrogen followed by the conversion of the gas to ethanol using homoacetogenic microorganisms. Methods for such conversion are known from U.S. Pat. No. 7,285,402 B2, US 20110059499 A1, US 20090215163 A1, and others.
Typically the substrate gas for carbon monoxide or hydrogen conversions is derived from a synthesis gas (syngas) from the gasification of carbonaceous materials, reforming of natural gas and/or biogas from anaerobic fermente4rs or from off streams of various industrial methods. The gas substrate contains carbon monoxide, hydrogen, and carbon dioxide and usually contains other components such as water vapor, nitrogen, methane, ammonia, hydrogen sulfide and the like. (For purposes herein, all gas compositions are reported on a dry basis unless otherwise stated or clear from the context.)
Production of ethanol from the substrate gas by these methods requires significant amounts of hydrogen and carbon monoxide. For instance, the theoretical equations for the conversion of carbon monoxide and hydrogen to ethanol are:6CO+3H2O.C2H5OH+4CO2 6H2+2CO2.C2H5OH+3H2OAs can be seen, the conversion of carbon monoxide results in the generation of carbon dioxide. The conversion of hydrogen involves the consumption of hydrogen and carbon dioxide, and this conversion is sometimes referred to as the H2/CO2 conversion. For purposes herein, it is referred to as the hydrogen conversion.
Syngas fermentation processes suffer from the poor solubility of the gas substrate, i.e., carbon monoxide and hydrogen, in the liquid phase of the fermentation menstruum. Munasinghe, et al., in Biomass-derived Syngas Fermentation in Biofuels: Opportunities and Challenges, Biosource Technology, 101 (2010) 5013-5022, summarize volumetric mass transfer coefficients to fermentation media reported in the literature for syngas and carbon monoxide in various reactor configurations and hydrodynamic conditions. As a result biofermentation processes for the production of ethanol will require large volumes of fermentation liquid. For example commercial scale plants, those with production capacities of 55 million gallons or more, will require fermentation zones that utilize vessels holding a million gallons or more of the fermentation liquid. Moreover, the fermentation processes will need to be operated in a continuous manner for extended periods of time.
To maintain the efficiency of producing ethanol by such fermentation zones there is a need to maximize the production of C2 oxygenated products while minimizing the production of higher carbon chain products such as C4, C6, C8 and higher organic acids or alcohols. The known methods seek to accomplish this efficiency by utilizing homoacetogenic bacteria that have a very high degree of selectivity for the production of C2 products. By their nature the homoacetogenic organisms that convert the gas substrate to ethanol do not have the pathways to make these longer carbon chain products.
The ability of homoacetogenic organisms to survive with minimal media on the CO and H2 substrate under anaerobic condition provides a protection against many biological contaminants that require much different environments. However, the size and scale of the fermentation zones and overall facilities necessary for the production of ethanol on a commercial basis precludes axenic operation of the facilities. As a result microbial contamination will inevitably occur at some point and can degrade the production by producing such higher chain byproducts which in turn severely reduces the yield of ethanol or other desirable products.
While there are many potential contaminants, one common class of potential contaminants will produce butyric acid, butanol and other longer chain organic acids or alcohols. Microorganisms that produce such compounds as part of their primary metabolism are referred to as butyrogens. There are many classes of butyrogens. One major class utilizes carbohydrates and other carbon compounds such as amino acids, lipids, etc. Another class of butyrogens uses syngas and yet another class of butyrogens can utilize ethanol and acetate with transferase enzyme pathways. Since, for the reasons previously mentioned, it is not possible to operate such large fermentation axenically, all of these butyrogen contamination sources will exist.
Once the butyrogen contamination takes hold in a large scale fermentation vessel it can destroy the commercial viability of the process by shifting feed conversion from desired products and making product recovery impractical. Designing product recovery facilities for wide variations in composition and concentration of liquid compounds would add prohibitive cost. The large volume of the fermentation liquid and the time to incubate the microorganisms to production concentrations make flushing and restarting of the facility impractical as well.
In contrast, conventional ethanol plants, such as corn ethanol plants, operate a plurality of batch reactors and thus inherently limit the time that competitive microorganisms have available for population increase. Indeed, often the duration of the batch fermentations is based upon the ethanol titer and the concentration of undesired, higher alcohols. Moreover, the fermenters can be sterilized between batches to eliminate the presence of undesired microorganisms.
Therefore methods are sought to eliminate or inhibit the growth of butyrogens in a large scale fermentation zone without disrupting the ongoing production of ethanol or other products such as acetic acid, propanol, or propionic acid from such a fermentation zone.